The present invention relates to a CRT (cathode-ray tube) bulb glass containing PbO for X-ray absorption and, in particular, to such an X-ray absorbing glass which is useful for a funnel part or a neck part of a color CRT bulb.
A CRT bulb comprises a panel part having an inner surface for forming a fluorescent screen thereon, a neck part for supporting an electron gun therein, a funnel part connecting the panel part and the neck part, and a stem sealing an end portion of the neck part. These parts are made of glass having properties individually adaptable thereto.
In operation of the CRT, the electron gun emits an electron beam which, in turn, produces, upon impinging onto the fluorescent screen, a visible light together with X-ray radiation. The X-ray radiation is harmful for human body and must be reduced to an acceptable level.
To this end, a conventional CRT bulb is usually made of glass having a high X-ray absorptivity, which contains lead oxide (PbO). For example, the neck part for supporting the electron gun is made of glass containing 20-40 weight % Pbo of a high X-ray absorptivity. The glass has an absorption coefficient not smaller than 80 cmxe2x88x921 with respect to the X-ray having a wavelength of 0.6 angstrom.
The funnel part is made of glass containing 10-30 weight % PbO to thereby have an absorption coefficient not smaller than 40 cmxe2x88x921 with respect to the X-ray having a wavelength of 0.6 angstrom.
Conventionally, as one approach to disposal of the CRT, it is practiced to dump the CRT under the ground after it is crushed into pieces. However, PbO contained in the glass may possibly dissolve out or be leached in water such as rain, moisture in the atmosphere and/or in the ground during a long period of time. It is a great concern that the lead leached would unfavorably result in environmental pollution.
In order to avoid the environmental pollution caused by lead leached from the CRT bulb glass into the ground, it is a measure to decrease the content of PbO in the CRT bulb glass. However, if PbO is decreased by more than a predetermined amount, it is impossible to obtain a sufficient X-ray absorptivity.
To compensate for this, it may be considered to make the glass contain a large quantity of SrO or BaO which exhibits a relatively high X-ray absorptivity. In this approach, however, a liquid-phase temperature of the glass increases so that the glass tends to be devitrified, resulting in a difficulty in shaping the glass.
Therefore, it is an object of the present invention to provide an X-ray absorbing CRT bulb glass which, without decreasing PbO content in comparison with conventional CRT bulb glass, can suppress its PbO content to dissolve out even if it is dumped under the ground.
As a result of various experiments, the present inventor has found out that addition of a predetermined amount of Fe2O3 into lead glass can suppress the lead content to leach into water. This could be understood to be based on the reason that, upon addition of Fe2O3, a network structure of the glass is electrically strengthen by Fe ions to make gaps in the network structure narrow so that it Pb ions hardly go out through the gaps.
The present invention is based on the novel knowledge.
According to the present invention, there is provided an X-ray absorbing CRT bulb glass containing 10-40 weight % PbO and having an absorption coefficient not smaller than 40 cmxe2x88x921 with respect to an X-ray having a wavelength of 0.6 angstrom, wherein the glass contains 0.06-10 weight % Fe2O3.
According to an aspect of the present invention, there is provided a CRT bulb glass containing 20-40 weight % PbO and having an absorption coefficient not smaller than 80 cmxe2x88x921 with respect to X-ray having a wavelength of 0.6 angstrom, wherein the glass contains 0.06-10 weight % Fe2O3, and is used for a neck glass.
According to another aspect of the present invention, there is provided a CRT bulb glass containing 10-30 weight % PbO and having an absorption coefficient not smaller than 40 cm31 1 with respect to X-ray having a wavelength of 0.6 angstrom, wherein the glass contains 0.06-10 weight % Fe2O3, and is used for a funnel glass.
The X-ray absorbing CRT bulb glass containing PbO of the present invention contains 0.06-10 weight % Fe2O3, preferably 0.1-10 weight %, and more preferably 0.5-5weight %, and is thereby possible to maintain a high X-ray absorptivity and to considerably suppress lead content to dissolve out. It is noted that Fe2O3 causes no environmental pollution even if it is introduced into the ground.
The reason why the content of Fe2O3 is limited as mentioned above is as follows. When the content is less than 0.06 weight %, it is impossible to obtain a remarkable effect of suppressing lead from leaching. On the other hand, the content of more than 10 weight % makes it difficult to shape the glass due to devitrification of the glass. Further, if the content of Fe2O3 is increased, an infrared transmittance of a molten glass is decreased, so that it becomes difficult to uniformly or homogeneously melt the glass in a glass melting tank. Especially, in case where the glass is melted in a large-sized tank, melt of the glass is degraded so that stones or other defects tend to be produced in the molten glass. Thus, it is desirable to suppress the Fe2O3 content to a level as small as possible.
When the CRT bulb glass of the present invention is used as a neck glass, it is preferable to contain 20-40 weight % PbO and to have a high absorption coefficient not smaller than 80 cmxe2x88x921 with respect to X-ray having a wavelength of 0.6 angstrom. When the content of PbO is less than 20 weight %, it is difficult to obtain a sufficient X-ray absorptivity as the neck glass. On the other hand, the content of more than 40 weight % is unfavorable because the viscosity of the glass becomes excessively low, resulting in difficulty in forming a neck. In addition, when the absorption coefficient is less than 80 cmxe2x88x921 with respect to X-ray having a wavelength of 0.6 angstrom, the CRT bulb glass is not suitable for the neck glass because X-ray quantity transmitted therethrough becomes excessive, resulting in an adverse influence on the human body.
A composition suitable as the CRT neck glass essentially consists of, by weight percent, 38-58% SiO2, 0-5% Al2O3, 20-40% PbO, 0-5% MgO, 0-6% CaO, 0-9% SrO, 0-9% BaO, 0-5% Na2O, 6-15% K2O, 0-1% Sb2O3, 0-10% TiO2, and 0.06-10% Fe2O3, and preferably 40-55% SiO2, 0.5-4% Al2O3, 25-38% PbO, 0-3% MgO, 0-4% CaO, 0-7% SrO, 0-7% BaO, 0-4% Na2O, 8-14% K2O, 0-0.6% Sb2O3, 0.1-5% TiO2, and 0.5-5% Fe2O3.
When the CRT bulb glass of the present invention is used as a funnel glass, it is preferable to contain 10-30 weight % PbO and to exhibit a high absorption coefficient not smaller than 40 cmxe2x88x921 with respect to X-ray having a wavelength of 0.6 angstrom. When the content of PbO is less than 10 weight %, it is difficult to obtain a sufficient X-ray absorptivity as the funnel glass. On the other hand, the content of more than 30 weight % is unfavorable because the viscosity of the glass becomes excessively low, resulting in difficulty in forming a funnel.
In addition, when the absorption coefficient is less than 40 cmxe2x88x921 with respect to X-ray having a wavelength of 0.6 angstrom, the CRT bulb glass is not suitable for the funnel glass because the X-ray quantity transmitted therethrough becomes excessive, resulting in an adverse influence on the human body.
A composition suitable as the CRT funnel glass essentially consists of, by weight percent, 48-58% SiO2, 0.5-6% Al2O3, 10-30% PbO, 0-5% MgO, 1-6% CaO, 0-9% SrO, 0-9% BaO, 3-9% Na2O, 4-11% K2O, 0-1% Sb2O3, 0-10% TiO2, and 0.06-10% Fe2O3, and preferably 49-57% SiO2, 1-5% Al2O3, 15-27% PbO, 0-4% MgO, 2-5% CaO, 0-4% SrO, 0-4% BaO, 4-8% Na2O, 5-10% K2O, 0-0.6% Sb2O3, 0.1-5% TiO2, and 0.5-5% Fe2O3.
In the present invention, it is possible to contain other components in addition to those mentioned above as far as characteristics of the glass are not deteriorated. For example, ZrO2, CeO2, Li2O, and V2O5 may be added up to 3 weight % in total. It should be avoided to add those components such as Cr2O3, CdO, and As2O3 which may possibly cause environmental pollution when dumped in the ground.